1.Field of the Invention
The present invention relates to improvements made to a beacon lantern and a lens for the same using LEDs as light source.
2. Prior Art
A variety of attempts have been made to use an LED as light source of a beacon lantern because of its small power consumption. Especially, automatic dispatching lanterns combined with solar batteries have been used.
Because an LED has a limited light-emitting energy by itself, a lens is incorporated in front or the LED itself to converge a beam of light and increase luminance thereof. However, if a plurality of LEDs of large convergence rate are used, beams of light are not distributed in all horizontal directions evenly. Instead, there will be strong and weak beams of light distributed horizontally. Therefore, in order to evenly distribute light horizontally, it is necessary to use LEDs of a relatively larger divergence angle and to eventually converge beams of light horizontally through a cylindrical fresnel lens.
In order to make a thin cylindrical fresnel lens, the lens includes a main lens part and a multi-layered annular part provided above and below the main lens part, and the light-emitting surface of which corresponds to the lens surface and consists of a curved surface requiring precision in forming. FIG. 14 shows a schematic view of such lens.
Cast glass has been used as a material for a cylindrical fresnel lens for waterway beacon lighting equipment. However, because of various reasons such as mold precision, casting, mold releasing and rolling process, it has been impossible to manufacture a high performance lens.
Recently, high performance cast cylindrical lenses are produced by the use of a plastic injection molding method. But this method comes with a limitation as to securing a proper thickness of the cylindrical lens. Either way, molds for the casting require high precision and are expensive. In the case of plastic lens, a certain volume has to be continuously provided in order to stabilize the quality, thereby resulting in high manufacturing cost.
Demand for waterway beacon lenses is very small compared to general commodities. The combination of special attributes of the lens, expensive mold, limited production volume and severe performance requirements have made development of a new lens for a demanded purpose very difficult. As a result, a lantern using such cylindrical fresnel lens has been expensive and there has been a great transmission loss of light.
The thinner the lens, the less light is lost through the lens. Therefore, the smaller the pitch between the rings of the fresnel lens, the thinner the lens can be made. Recent improvement in machining precision of molds and progress in plastic materials have made a pitch of 0.1 mm possible. Also, if the pitch is made small, sufficient precision can be obtained even if the vertical section of the light-emitting surface is processed linearly, i.e., in a prism shape, instead of a curved line, thereby making designing and machining of a mold very easy.